Photooxidation oxygen, polymeric

Among the more interesting applications of polymer Rose Bengal is that of a sensitizer in studying the oxidation of other polymeric substrates [301]. Rose Bengal immobilized on Sepharose has been reported as a sensitizer for protein photooxidation [302], The oxygen uptake by the amino acids cysteine, hisitidine, methionine, tryptophan, and tyrosine was reported to be about 20% as much from the immobilized dye as from the free dye in aqueous solution. 

The polymeric pyrrolic autoxidation products probably result from the oxidized monomeric systems, which are analogous in structure to those isolated from photooxidation and peroxide oxidation reactions. Thus, for example, analysis of the products of the autoxidation of 1-methylpyrrole (Scheme 47) would suggest that 1 -methyl-A3-pyrrolin-2-one (153) is initially formed from a radical reaction of the pyrrole with triplet oxygen. This reaction sequence should be compared with that proposed for the oxidation of pyrroles with hydrogen peroxide (Scheme 50), which yields (181), (182) and (183) as the major isolable products. The acid-catalyzed reaction of a pyrrole with its oxidation product e.g. 153) also results in the formation of polymeric material and the formation of pyrrole black is probably a combination of oxidation and acid-catalyzed polymerization processes. 

Both curves have similar shapes and it is clear that the photooxidation process is controlled by the diffusion of oxygen into the rose bengal sites in the polymer solution. These results suggest that when a snail amount of rose bengal is attached to the polymeric backbone (P-RB-51, 102 152 305) the quantum yield of of singlet oxygen formation is essentially the same (about 0.38). 

The first comprehensive study was made by Taylor and Blacet,20 who studied the nature of the products at 3130 A. at temperatures from 60 to 140°C. and in the presence of large pressures of oxygen (10-124 mm.) Table V. Conditioning of the cell walls by photooxidation or by boric acid helped to reduce the considerable scatter in their experimental results. Analyses, performed mass spectrometrically, gave the main products as carbon monoxide, carbon dioxide, formaldehyde, and water, while minor products were methanol, acetic acid, and a polymeric substance that gradually accumulated on the walls. No methane, ethane, or formic acid was deteoted. 

A membrane-induced structure-reactivity trend that may be exploited to achieve selective processes has been recently observed in polymeric catalytic membranes prepared embedding polyoxotungstates, W(VI)-oxygen anionic clusters having interesting properties as photocatalysts, in polymeric membranes [17]. These catalytic membranes have been successfully apphed in the photooxidation of organic substrates in water providing stable and recyclable photocatalytic systems. 

The polymers obtained by polymerization in the presehce of metal catalysts contain metal residues which cannot be removed so readily. It is also well known that transition metal ions act as sensitizers for the photooxidation of polyolefins (29). Kujirai et al. (30) found that photodegradation of polypropylene depends on the oxygen concentration and on the residues of the polymerization catalyst, and they concluded that oxidative photodegradation is sensitized by the initiator metal residues (ash). Very recently Scott (31) used transition metal ions as sensitizers to develop photodegradable polymers. 

In the case of simple fulvenes exposure to atmospheric oxygen leads to amorphous polymeric products which may be explosive, and whose structures are generally unknown. Autoxidation may be accompanied by photooxidation, but again not much is known about the processes. It has been shown that photooxidation of some simple fulvenes gives rise to lactones (and other products)[226,227] ... 

It is well known that molecular oxygen inhibits free-radical polymerization by scavenging the initiator radicals, which not only reduces the polymerization rate but also affects the mechanical, optical, and structural properties of the cured systems. The mechanism of O2 inhibition is represented in Figure 6(a). As a result of a photooxidation reaction, peroxy radicals (or hydroperoxides or alcoxy radicals) are generated, which are less reactive toward monomer to initiate... 

The type I process gives two free radicals, one polymeric and one small acyl radical. The polymeric radical can undergo a rearrangement known as )S-scis-sion which results in a break of a C—C bond in the backbone of the polymer and a consequent reduction in molecular weight. The type II reaction, however, is the major photodecomposition process that causes chains to break. In the presence of oxygen, both radical sites can induce photooxidation processes which cause chain degradation over a longer time scale. 

The rate of photooxidation depends on the namre of polymer, on the polymeric layer thickness, on oxygen solubility and diffiision rate, as well as on the crystallinity degree or on the branching degree of macromolecular chain [11] and on the level of irradiance [16]. 

---